Turbine engines are used in many industrial applications in which a mechanical driving force is needed to produce a useful result. One of the most visible applications is in the generation of electricity where so-called “green” resources such as hydro-electric, wind turbine, or solar photovoltaic power are not available. Even where such resources are available, they typically lack the flexibility to rapidly increase or decrease the supply of electricity as demand fluctuates, whereas turbine engines do have such flexibility. Accordingly, although green resources are becoming more prevalent, and may be expected to assume an ever-increasing share of electrical power generation, it is anticipated that the use of turbine engines will continue into the indefinite future. That being so, there is a continuing need to continue to automate, streamline, and increase the efficiency of systems in which turbine power plays a significant role. One of the areas in which automation and efficiency may be enhanced is in the planning and scheduling of maintenance that may be due to causes other than normal wear and tear. Even new or newly refurbished turbine engines may have their life expectancies shortened by the incidence of unexpected damage to rotor or stator blades caused by the ingestion of foreign objects, or by latent manufacturing or assembly defects that become evident only when the engine is being operated.
Embodiments of the present invention generally relate to rotor blades in combustion and steam turbine engines, both of which are extensively used in the generation of electrical power, and more particularly to systems and methods for monitoring the health of such blades. Rotor blades or rotating blades are used in many devices with several examples including compressors, turbines, and engines. A gas (or combustion) turbine engine typically includes a compressor section, a combustor section, and a turbine section. The compressor and the turbine sections generally include rows of blades that are axially stacked in stages. Each stage includes a row of circumferentially-spaced stator blades, which are fixed, and a row of rotor blades, which are attached to and rotate with a central axis or shaft. In operation, the compressor rotor blades rotate and, acting in concert with the stator blades, compress a flow of air. The supply of compressed air then is used in the combustor to combust a supply of fuel. The resulting flow of hot expanding gases from the combustion, i.e., the working fluid, is expanded through the turbine section of the engine. The flow of working fluid through the turbine blades turns the central shaft that the turbine blades are attached to and that, in turn, causes the rotor blades to rotate. A steam turbine engine does not require a compressor section, as the transformation of water to steam provides the expanding gas necessary to rotate the turbine and provide torque to turn the shaft.
In either case, the energy contained in the fuel is converted into the mechanical energy of the rotating shaft, which may be attached to the coils of an electrical generator such that electrical power is generated. During operation of combustion turbine engines, because of the extreme temperatures of the hot-gas path, the velocity of the working fluid, and the rotational velocity of the engine, compressor blades, which generally include both the rotating rotor blades and the fixed stator blades, become highly stressed with extreme mechanical and thermal loads. Steam turbine engines experience similar mechanical and thermal stresses to rotor blades in the turbine section of the engine.
Various factors adversely affect health of the rotor blades and lead to fatigue, stress, and ultimately cracking of the rotor blades. These include not only the mechanical and thermal loads mentioned earlier, but also any damage that may occur when external objects or debris are inadvertently sucked into the compressor of an operating engine, or internal components become dislodged. Such damage, frequently referred to as foreign object damage (“FOD”) or domestic object damage (“DOD”), can chip, bend, or weaken one or more rotor blades such that the blade or blades will fail before their expected lifespan in the absence of such damage. In some instances, blades may be affected in a manner that causes the tip of a blade to scrape or rub against the interior wall of the engine casing. Once a blade tip begins to rub, the blade may develop cracks or other damage that can be used to predict failure. By monitoring the health of each blade in a turbine engine, unexpected failures may be avoided or substantially mitigated, and maintenance can be scheduled to correct blade defects before a catastrophic failure occurs.
Given the extreme conditions of operation for rotor blades, it is important for rotor blade health to be monitored closely. In many cases, blade failures may be predicted and avoided if data concerning blade damage is accurately collected and monitored. Such data may include strain levels and/or crack formation/propagation in certain highly stressed areas on the blade. Blade health monitoring is typically done using sensors embedded in the casing of a turbine engine that precisely measure the passage of the tip of a rotating blade. By using a plurality of sensors placed at precise locations around the circumference of a rotor stage, vibrational characteristics of each blade can be measured, recorded, and analyzed, and determinations made about the health of each blade.
It is well understood that blades in an operating turbine have resonant vibration frequencies that depend on a number of parameters, including the characteristics of the blade material, blade size, rotational speed, and a variety of other factors. In this context, the term “resonant vibration frequency” refers to the vibration frequency of a blade under operational conditions, and does not refer to the rotational frequency of the shaft to which the blade is attached. Resonant vibration frequencies of blades in an undamaged engine can be determined empirically and monitored during start-up, shut-down, and normal operation. A change in the resonant frequencies of a blade is usually indicative of damage to the blade, and such frequency variations may be regarded as warning flags that a blade's health may be deteriorating. Once a blade develops a crack, changes in the resonant frequencies of the blade can be used to track the progression of the crack. As a crack grows in length, the time to blade failure will decrease, and predictions regarding remaining blade life may be made, based upon the rate of growth of the crack. Thus, with adequate monitoring and the use of algorithms to analyze blade vibration data, a blade's life expectance may be estimated, and appropriate steps be taken to replace the blade without adversely impacting other system operations.